This invention relates to hollow nylon fibers and more particularly to a method of making such fibers from microporous membranes.
Microporous membranes are well known in the art. For example, U.S. Pat. No. 3,876,738 to Marinaccio and Knight (1975) describes a process for preparing a microporous membrane, for example, by quenching a solution of a film forming polymer in a non-solvent system for the polymer U.S. Pat. Nos. 4,340,479 and 4,340,480 to Pall describe similar processes.
The microporous membranes can be formed into hollow fibers having a continuously extending hollow portion over their whole length by generally extruding a spinning solution, that is the solution of the film forming polymer, from an annular slit to form a sheath solution which is quenched and simultaneously introducing a core liquid into the inside hollow portion of the sheath solution. In a wet spinning process, the spinning solution is directly extruded into the coagulating quench bath by immersing the orifice therein. In the dry jet-wet spinning process, the spinning solution is extruded from the orifice and the resulting filament is introduced into the coagulating bath after being passed through a gaseous space between the orifice and the bath.
Joh et al., U.S. Pat. No. 4,284,594, describes a method of manufacturing a hollow fiber in which a spinning solution of a cellulose ester dissolved in an organic solvent is extruded from an annular slit and simultaneously a core liquid is introduced into the inside hollow portion of the extruded spinning solution, in which the core liquid is a hydroxycarboxylic acid, a derivative of the hydroxycarboxylic acid, monoterpene, or mixture, or a solution of such materials or mixtures. A similar procedure is described in Joh et al., U.S. Pat. Nos. 4,322,381 and 4,323,627, in which the polymer is a cellulose ester, polymethylmethacrylate, polyvinyl chloride, a polybenzyl glutamate, polyacrylonitrile or an acrylonitrile copolymer and the core of liquid is an aqueous salt solution having a salt content of 15-50% with or without sufficient alkali or acid present in the aqueous salt solution to affect controlled hydrolysis of the polymer.
Arisaka et al., U.S. Pat. No. 4,127,625, describes a method of producing asymmetric hollow fibers by spinning a solution of a fiber forming cellulose derivative through the outer tube of a double tube-type spinneret into an aqueous coagulating bath to form a hollow fiber and simultaneously injecting an aqueous salt solution through the inner tube of the spinneret to contact the interior of the hollow fiber.
In the formation of nylon hollow fibers, the solution of nylon (known as the "dope") is extruded through the annular orifice and the solids in the dope are made to congeal by contact with quench solutions both inside and outside of the dope annulus. The resulting hollow fiber generally has an outside diameter which is smaller than the diameter of the orifice plate as a result of a drawdown phenomenon which occurs just past the spinneret. At this point, the nylon dope is still in a fluid state and both the force of gravity and tensile forces act on the nascent fiber before the fiber has developed sufficient structural rigidity to resist a change in size. The combination of these forces act on the extruded dope to cause the extruded dope to collapse toward the central longitudinal axis of the extruded dope sheath. As a result of the drawdown, the final fiber diameter is often less than 50% of the diameter of the orifice through which the fiber has been extruded. Where it is desirable to make large diameter fibers, for example more than 40 mils outside diameter, it can be very difficult to make the desired fibers because even the use of large diameter orifices results in drawdown effects that ultimately yield fibers only slightly larger than those made from smaller orifices.
It is the object of this invention to provide a method of producing hollow nylon fiber products in which the drawdown phenomenon is substantially eliminated such that large diameter hollow fibers can be realized. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.